1. Field of the Invention
The present invention relates to a design method for an objective lens installed in a recording and reproducing apparatus of a multiple wavelengths optical system using plural kinds of monochromatic lights which is compatible with optical storage medium of different types such as compact discs (CD) including a CD-R and digital versatile discs (DVD), an objective lens, a general-purpose multiple wavelengths lens, a multiple wavelengths optical system, an optical head, and an optical disc apparatus.
2. Related Background Art
Conventionally, a compatible optical disc apparatus capable of reproducing different types of optical discs such as a CD and DVD has been proposed. The CD and DVD (which will be hereinafter collectively called an optical disc) both are provided with transparent substrates, and there is provided an information surface on one side thereof. The optical disc has two transparent substrates combined together, and their information surfaces face each other, or the information surface faces the other transparent substrate as a protection substrate. To reproduce information signals stored in the above optical disc, the optical disc apparatus condenses a laser beam from a light source on the information surface of the optical disc through the transparent substrates. As detailed later, a wavelength of the laser beam differs between a CD and DVD. The optical disc apparatus uses an objective lens for condensing the laser beam. Here, the thickness of the transparent substrate provided with the information surface differs according to a type of the optical disc (a difference in a laser beam wavelength). While the transparent substrate of a CD is 1.2 mm in thickness, that of a DVD is 0.6 mm. For the optical disc apparatus to reproduce optical discs of different types, it is required to condense the laser beam on the information surface while the thickness of the transparent substrate differs with the type of the optical disc. Besides, a new optical disc apparatus using a blue laser of approximately 400 nm wavelength for reproducing information is recently proposed. Therefore, it is desirable for the optical disc apparatus to be compatible with the new optical disc in addition to the CD and the existing DVD.
One approach for the above optical disc apparatus is to provide a pickup with objective lens for different types of optical discs to change the objective lens in accordance with the type of the optical disc in use, or to provide pickups for different types of the optical discs to change the pickups in accordance with the type of the optical disc in use. However, for miniaturization and cost reduction, it is preferable to have a single objective lens usable for any types of the optical disc.
Known as this kind of an objective lens is one disclosed in Japanese Patent Application Laid-Open No. H09-145995, for example. A lens surface of the objective lens disclosed therein is radially sectioned into more than two loop zones, and every other loop zonal lens surfaces and the other every other loop zonal lens surfaces are different in refracting power. For a laser beam of the same wavelength, the every other loop zonal lens surfaces condense the laser beam on the information surface of the optical disc (DVD) provided with the thin transparent substrates (0.6 mm), and the other every other zonal lens surfaces condense the laser beam on the information surface of the optical disc (CD) provided with the thick transparent substrates (1.2 mm), for example.
Another example is one disclosed in Japanese Patent Application Laid-Open No. 2000-81566 (U.S. Pat. No. 6,118,594). It discloses the optical disc apparatus using a laser beam of the shorter wavelength (635 nm or 650 nm) for a DVD having the thinner transparent substrates while using a laser beam of the longer wavelength (780 nm) for a CD having the thicker transparent substrates. The optical disc apparatus is provided with the objective lens used in common for those laser beams. The objective lens has a diffractive lens structure having a plurality of minute loop zonal steps thickly formed on one side of a refractive lens having a positive refractive power. The diffractive lens structure is designed so that diffracted light of a laser beam having the shorter wavelength, and diffracted light of a laser beam having the longer wavelength, are condensed on the information surface respectively for the DVD provided with the thinner transparent substrate, and for the CD provided with the thicker transparent substrate. The lens is also designed to condense diffracted light having identical diffractive order for both the diffracted lights. The reason why a laser beam having the shorter wavelength is used for the DVD is because a storage density of the DVD is higher than that of the CD, thus requiring a smaller beam spot. As well known, the diameter of an optical spot is proportional to the wavelength and inversely proportional to the numerical aperture (NA).
There is also disclosed an objective lens of a loop zonal phase correction lens type provided with a loop zonal phase shifter formed on the lens surface thereof in Japanese Patent Application Laid-Open No. 2001-51192. First, a lens surface designed to have no wavefront aberration for a laser beam having wavelength λ1 of 640 nm is set as a reference. A surface of the objective lens is then radially sectioned into plural loop refractive zones, which are formed to have predetermined steps (i-th step from a center of the lens is referred to as di) from the reference lens surface. Due to the step di, each of the refractive surfaces allows the laser beam of the DVD to phase-shift by integral multiple mi of the wavelength λ1 against the reference lens surface, thereby reducing wavefront aberration in a CD system.
In any of the above conventional techniques, it is possible to use a common objective lens for both DVD and CD. It eliminates the need for changing members including an objective lens for each use of a DVD or CD, which is effective in reducing costs and simplifying the structure.
However, the technique disclosed in Japanese Patent Application Laid-Open No. H09-145995 uses different loop zonal lens surface of the objective lens from a DVD to CD. Accordingly, large area remains ineffective for an incident laser beam, which extremely lowers light use efficiency.
Also, the technique disclosed in Japanese Patent Application Laid-Open No. 2000-81566 (U.S. Pat. No. 6,118,594) has a problem that diffractive efficiencies for different wavelengths are unable to become 100% at the same time because it uses the diffracted light by the diffractive lens structure. In the diffractive lens, for a laser beam having the shorter wavelength (635 nm or 650 nm) used for a DVD and for a laser beam having the longer wavelength (780 nm) used for a CD, the diffractive efficiency is designed to become 100% at intermediate wavelength between the two, in order to make the diffractive efficiency well balanced for the laser beams in use. Besides, the technique requires minute steps to be formed on the lens surface for the diffraction lens structure, which is vulnerable to processing error. In a case where the diffractive structure fails to be formed as designed, it causes a decrease in the diffractive efficiently. When the diffractive efficiency decreases or when it does not reach 100%, it means incapability of condensing all the incident light on the information surface on the transparent substrate of the optical disc, which results in the light loss.
Further, while having the high light use efficiency, the loop zonal phase correction lens type as disclosed in Japanese Patent Application Laid-Open No. 2001-51192 also has a problem. It sets the lens surface designed to have no wavefront aberration for the DVD laser beam as a reference surface, and forms the refractive surfaces in di step, mi integral multiple of the wavelength λ1 of the DVD laser beam, below the reference lens surface, in order to reduce the wavefront aberration for the CD laser beam. However, setting the reference for the DVD and then merely forming steps from there does not sufficiently decrease the wavefront aberration for the CD laser beam.